The production of crude neopentyl glycol (NPG) is accomplished by well-known technology wherein formaldehyde and isobutyraldehyde undergo the aldol reaction in the presence of a basic catalyst. The aldol product, hydroxypivaldehyde, is hydrogenated in a conventional manner to produce a crude NPG product. The crude NPG typically contains various impurities, mainly NPG-isobutyrate and the self-condensation product of hydroxypivaldehyde. The NPG-isobutyrate, which boils at a temperature very close to the boiling point of NPG, is impractical to remove by conventional distillation. The self-condensation product of hydroxypivaldehyde, although easy to separate from the NPG by conventional distillation, represents a yield loss. These impurities are readily removed by saponification with caustic, which recovers NPG from the impurities and at the same time renders the impurities non-volatile. However, it is well known that the caustic-containing mixture is unstable at elevated temperatures, actually resulting in a less pure product when the caustic-treated NPG is distilled away from the caustic in a conventional distillation. Until now, no simple method has existed whereby the NPG could be recovered from the caustic.
U.S. Pat. No. 2,895,996 to Wright and Hagemeyer teaches that the saponified crude NPG may be subjected to a steam sublimation, whereby the NPG is recovered in an aqueous stream. The steam sublimation is conducted under reduced pressure (2-500 mmHg) and minimum temperature (60.degree.-150.degree. C.) in order to avoid the caustic-catalyzed decomposition of the NPG. The water, which may comprise 60-93% of the NPG-containing overhead stream from the sublimator, must be removed from the NPG by distillation. This results in a very energy-intensive process. In addition, some NPG must remain in the residue from the steam sublimation, and this represents a yield loss.
U.S. Pat. No. 3,939,216 to Wright describes an isobutyraldehyde extraction process for use on the caustic residue resulting from the following process: The aldol is caustic-catalyzed and the caustic-containing aqueous phase is separated from the hydroxypivaldehyde (HPA)-containing organic phase. The organic phase is hydrogenated, saponified with caustic to remove impurities, and then subjected to a steam-sublimation step. The residue from the steam-sublimation step is combined with the aqueous phase from aldol, and then the NPG is extracted with isobutyraldehde. The patent specifically limits the extraction feed to the combined streams, rather than the caustic residue stream only.
U.S. Pat. No. 4,021,496 to Wright also refers to the caustic-catalyzed aldol, wherein the caustic-containing aqueous stream is decanted from aldol and combined with the caustic residue from the steam-sublimation step. It claims an isobutyraldehyde (ibal) extraction of NPG from an "aqueous NPG-containing" stream and that the ibal can be used subsequently in aldol.
U.S. Pat. No. 4,038,329 to Palmer teaches a method for the purification of NPG made in the presence of an alkaline catalyst, whereby the NPG is extracted with a solvent such as isobutanol from the crude reaction product and then distilled under atmospheric pressure. Here, sodium is present as the aldol catalyst rather than for saponification of esters. The extraction is performed on the bulk of the NPG (i.e., prior to any distillation or evaporation), and accordingly some sodium is contained in the NPG and would be present in the subsequent atmospheric distillation.